ASBOG Geologic Hazards 2 — Questions and Answers
Question 1: What is the difference between volcanic hazard assessment and volcanic risk assessment?
- Hazard assessment measures lava viscosity; risk assessment measures eruption frequency
- Hazard assessment evaluates the probability and potential impact of volcanic phenomena (lava flows, pyroclastic density currents, lahars, tephra); risk assessment combines hazard with vulnerability and exposure of human and built environment to estimate potential losses (Correct answer)
- Hazard assessment applies only to active volcanoes; risk assessment applies to all geologic hazards
- Hazard assessment is conducted by volcanologists; risk assessment is conducted only by insurance companies
Correct answer: Hazard assessment evaluates the probability and potential impact of volcanic phenomena (lava flows, pyroclastic density currents, lahars, tephra); risk assessment combines hazard with vulnerability and exposure of human and built environment to estimate potential losses
Volcanic hazard assessment maps the probability and spatial extent of volcanic processes; risk equals hazard multiplied by vulnerability and exposure — how many people and assets are in harm's way and how susceptible they are to damage.
Risk = Hazard × Vulnerability × Exposure. Volcanic hazard assessment maps different volcanic phenomena (lava flows, pyroclastic density currents/surges, lahars, tephra fall, volcanic gases, tsunamis from flank collapse) and their probability of occurrence and area of impact. Vulnerability describes the susceptibility of elements at risk (people, buildings, lifelines) to damage for a given hazard intensity. Exposure is the value (people, assets, critical infrastructure) within the hazard zone. Risk assessment integrates all three components to prioritize mitigation, evacuation planning, and insurance. The 1991 Pinatubo eruption demonstrated how combining hazard mapping with community evacuation saved tens of thousands of lives despite enormous ash fall and lahars.
Question 2: What is a lahar, and under what conditions do lahars remain dangerous long after a volcanic eruption has ended?
- A lahar is a volcanic bomb ejected from a cinder cone; it remains dangerous because it stays hot for weeks after eruption
- A lahar is a volcanic mudflow or debris flow of water and volcanic material; it can remain hazardous for years because loose volcanic debris on slopes is repeatedly remobilized by rainfall into rivers and valleys downstream of the volcano (Correct answer)
- A lahar is a submarine volcanic landslide; it remains dangerous because it generates repeated tsunamis for months
- A lahar is a volcanic gas cloud; it remains dangerous because of long-term carbon dioxide seepage from the edifice
Correct answer: A lahar is a volcanic mudflow or debris flow of water and volcanic material; it can remain hazardous for years because loose volcanic debris on slopes is repeatedly remobilized by rainfall into rivers and valleys downstream of the volcano
Lahars are volcanic mudflows composed of water mixed with pyroclastic debris; they can persist for years to decades as rainfall mobilizes unstabilized tephra deposits on volcano flanks into rivers, causing repeated floods and channel aggradation far from the volcano.
Lahars (a Javanese term) are rapid debris flows or mudflows composed of volcanic material and water. Primary lahars are generated during eruption (crater lake expulsion, ice and snow melt from pyroclastic flows). Secondary lahars form after eruption when heavy rainfall remobilizes unstabilized tephra and debris from volcano flanks and valleys. The 1991 Pinatubo eruption deposited vast quantities of loose pyroclastic material; secondary lahars have continued for decades, repeatedly inundating towns in Pampanga, Tarlac, and Zambales provinces as Typhoon-season rains mobilize the remaining deposits. Lahar hazard may persist for 10–30 years after major eruptions.
Question 3: In the context of earthquake ground shaking, what is site amplification and which type of site is most susceptible?
- Site amplification refers to the magnification of ground motion by local soil or rock conditions relative to a reference bedrock site; soft sediments (loose sand, saturated clay, bay mud) amplify shaking most due to their low shear-wave velocity and impedance contrast with underlying rock (Correct answer)
- Site amplification refers to strong ground motion near active faults where fault slip amplifies P-waves
- Site amplification is the increase in earthquake magnitude reported at seismic stations close to the epicenter due to proximity
- Site amplification refers to the concentration of soil failure (liquefaction) in thick alluvial deposits, not to shaking intensity
Correct answer: Site amplification refers to the magnification of ground motion by local soil or rock conditions relative to a reference bedrock site; soft sediments (loose sand, saturated clay, bay mud) amplify shaking most due to their low shear-wave velocity and impedance contrast with underlying rock
Site amplification is the local increase in ground shaking intensity due to trapping and resonance of seismic waves in soft sediment overlying bedrock; sites with soft, thick, low-velocity sediments (San Francisco Bay mud, Mexico City lake sediments) amplify ground motion most severely.
When seismic waves travel from hard bedrock into softer overlying sediment, their velocity decreases and their amplitude increases to conserve energy (conservation of seismic energy flux). This wave trapping and resonance can amplify ground shaking by factors of 5–20 compared to bedrock sites at the same distance from the earthquake. The 1985 Mexico City earthquake (Mw 8.1) caused severe building damage in the city despite occurring 350 km away because soft lake sediments amplified ground motion dramatically. The 1989 Loma Prieta earthquake similarly caused focused damage in the Marina District of San Francisco, built on bay mud. NEHRP site classes (A through E) quantify amplification based on average shear-wave velocity (Vs30).
Question 4: What is a tsunami, and how does the wave speed of a tsunami in the open ocean relate to water depth?
- A tsunami is a wind-driven ocean wave generated by offshore hurricanes; its speed is controlled by wave period and is independent of water depth
- A tsunami is a large ocean wave generated by rapid displacement of the seafloor (earthquake, submarine landslide, volcanic flank collapse); its speed in the open ocean follows v = √(gd), where g is gravitational acceleration and d is water depth — reaching ~800 km/h in the deep ocean (Correct answer)
- A tsunami is a harbor resonance wave caused by distant earthquakes; its speed is controlled by tidal forcing and varies with the position of the moon
- A tsunami is a standing wave trapped in a bay or harbor; its speed increases in shallow water as the seabed friction decreases
Correct answer: A tsunami is a large ocean wave generated by rapid displacement of the seafloor (earthquake, submarine landslide, volcanic flank collapse); its speed in the open ocean follows v = √(gd), where g is gravitational acceleration and d is water depth — reaching ~800 km/h in the deep ocean
Tsunamis are long-wavelength ocean waves generated by seafloor displacement; their speed is given by the shallow-water wave approximation v = √(gd), reaching ~800 km/h in deep ocean (4,000 m depth) and slowing to 30–50 km/h in shallow coastal water while growing in height.
Tsunamis behave as shallow-water waves even in the deep ocean because their wavelengths (100–500 km) are far greater than ocean depths. Their phase velocity is c = √(gd), where d is water depth. In 4,000 m depth, c ≈ 200 m/s (~720 km/h). As tsunamis enter shallow coastal water, their velocity decreases proportional to √d, but conservation of wave energy causes their height (amplitude) to increase dramatically (shoaling). This 'wave amplification' can transform a 0.5 m wave in the open ocean into a 10–30 m runup at the coast. The 2004 Indian Ocean tsunami crossed the ocean in hours and killed 228,000 people, demonstrating the hazard of trans-oceanic tsunami propagation.
Question 5: What is the difference between a debris flow and a rockfall in terms of geologic hazard characteristics?
- Debris flows are dry granular flows; rockfalls are water-saturated mass movements on gentle slopes
- Debris flows are fast-moving, water-saturated mixtures of sediment and organic material that travel in channels; rockfalls involve free-falling, bouncing, or rolling of individual blocks detached from a cliff or steep rock face — both are rapid and difficult to warn against (Correct answer)
- Debris flows are slow-moving (mm/year) slope failures; rockfalls are seismically triggered events only
- Debris flows occur only in volcanic terrain; rockfalls occur only in glacially eroded alpine terrain
Correct answer: Debris flows are fast-moving, water-saturated mixtures of sediment and organic material that travel in channels; rockfalls involve free-falling, bouncing, or rolling of individual blocks detached from a cliff or steep rock face — both are rapid and difficult to warn against
Debris flows are rapid, channelized flows of water-saturated sediment and organic debris; rockfalls involve the free fall or rolling of detached rock blocks from steep cliffs. Both are rapid mass movements but differ in mechanism, volume, and travel path.
The Varnes (1978) and Hungr et al. (2014) classification systems distinguish mass movements by material type, water content, and movement mechanism. Debris flows (rapid, water-saturated, channelized flows of poorly sorted material with > 50% coarse particles) travel at 1–20 m/s, follow drainage channels, and deposit lobate bouldery fans. Rockfalls involve the detachment of one or more rock masses from a steep cliff by tensile failure (frost wedging, seismic shaking, weathering), followed by free fall, bouncing, or rolling. Velocities can exceed 30 m/s. Both are difficult to predict precisely, require warning systems (rain gauges, extensometers), and are mitigated by catchment berms, nets, and slope stabilization.
Question 6: What is 'induced seismicity,' and which industrial activity has been most widely linked to significant induced earthquake sequences in the United States?
- Induced seismicity is vibration caused by heavy industrial machinery near active faults; mining operations for coal are the primary cause in the US
- Induced seismicity is earthquakes triggered by human activities; wastewater disposal (injection of produced water from oil and gas operations into deep disposal wells) has caused the most significant induced earthquake sequences in the central US since 2008 (Correct answer)
- Induced seismicity is seismic noise from wind farms interfering with seismograph recordings; it is a monitoring problem, not an actual earthquake hazard
- Induced seismicity is vibration from traffic and construction; urban infrastructure development is the primary cause globally
Correct answer: Induced seismicity is earthquakes triggered by human activities; wastewater disposal (injection of produced water from oil and gas operations into deep disposal wells) has caused the most significant induced earthquake sequences in the central US since 2008
Induced seismicity refers to earthquakes triggered by human activities; deep-well injection of wastewater (primarily produced water from oil and gas operations) into crystalline basement aquifers in the central US has been responsible for a dramatic increase in earthquake rates, including M5+ events in Oklahoma, Kansas, and Ohio.
The central and eastern US experienced a dramatic increase in seismicity from 2008–2015, coinciding with increased deep-well disposal of produced water from unconventional oil and gas development (hydraulic fracturing). The primary mechanism is pore pressure increase from fluid injection, which reduces the effective normal stress on pre-existing faults, allowing them to slip at lower shear stresses (Coulomb stress transfer). Oklahoma went from ~1–2 M3+ earthquakes per year to over 900 in 2015. Significant M5.0–5.8 induced earthquakes have caused building damage. Regulatory response included volume and pressure limits on disposal wells near active faults. Reservoir impoundment, geothermal drilling, and deep mining also cause induced seismicity.
ASBOG Geologic Hazards 2
What is the difference between volcanic hazard assessment and volcanic risk assessment?